void glGetActiveAttrib(GLuint program, GLuint index, GLsizei bufSize, GLsizei *length, GLint *size, GLenum *type, GLchar *name);
A vertex shader may use either built-in attribute variables, user-defined attribute variables, or both. Built-in attribute variables have a prefix of "gl_" and reference conventional OpenGL vertex attribtes (e.g., gl_Vertex, gl_Normal, etc., see the OpenGL Shading Language specification for a complete list.) User-defined attribute variables have arbitrary names and obtain their values through numbered generic vertex attributes. An attribute variable (either built-in or user-defined) is considered active if it is determined during the link operation that it may be accessed during program execution. Therefore, program should have previously been the target of a call to glLinkProgram(), but it is not necessary for it to have been linked successfully.
The size of the character buffer required to store the longest attribute variable name in program can be obtained by calling glGetProgram() with the value GL_ACTIVE_ATTRIBUTE_MAX_LENGTH. This value should be used to allocate a buffer of sufficient size to store the returned attribute name. The size of this character buffer is passed in bufSize, and a pointer to this character buffer is passed in name.
glGetActiveAttrib returns the name of the attribute variable indicated by index, storing it in the character buffer specified by name. The string returned will be null terminated. The actual number of characters written into this buffer is returned in length, and this count does not include the null termination character. If the length of the returned string is not required, a value of NULL can be passed in the length argument.
The type argument specifies a pointer to a variable into which the attribute variable's data type will be written. The symbolic constants GL_FLOAT, GL_FLOAT_VEC2, GL_FLOAT_VEC3, GL_FLOAT_VEC4, GL_FLOAT_MAT2, GL_FLOAT_MAT3, GL_FLOAT_MAT4, GL_FLOAT_MAT2x3, GL_FLOAT_MAT2x4, GL_FLOAT_MAT3x2, GL_FLOAT_MAT3x4, GL_FLOAT_MAT4x2, GL_FLOAT_MAT4x3, GL_INT, GL_INT_VEC2, GL_INT_VEC3, GL_INT_VEC4, GL_UNSIGNED_INT, GL_UNSIGNED_INT_VEC2, GL_UNSIGNED_INT_VEC3, GL_UNSIGNED_INT_VEC4, GL_DOUBLE, GL_DOUBLE_VEC2, GL_DOUBLE_VEC3, GL_DOUBLE_VEC4, GL_DOUBLE_MAT2, GL_DOUBLE_MAT3, GL_DOUBLE_MAT4, GL_DOUBLE_MAT2x3, GL_DOUBLE_MAT2x4, GL_DOUBLE_MAT3x2, GL_DOUBLE_MAT3x4, GL_DOUBLE_MAT4x2, or GL_DOUBLE_MAT4x3 may be returned. The size argument will return the size of the attribute, in units of the type returned in type.
The list of active attribute variables may include both built-in attribute variables (which begin with the prefix "gl_") as well as user-defined attribute variable names.
This function will return as much information as it can about the specified active attribute variable. If no information is available, length will be 0, and name will be an empty string. This situation could occur if this function is called after a link operation that failed. If an error occurs, the return values length, size, type, and name will be unmodified.
GL_INVALID_OPERATION is generated if program is not a program object.
GL_INVALID_VALUE is generated if index is greater than or equal to the number of active attribute variables in program.
GL_INVALID_VALUE is generated if bufSize is less than 0.
glGetProgram() with argument GL_ACTIVE_ATTRIBUTES or GL_ACTIVE_ATTRIBUTE_MAX_LENGTH.
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